1. Field of the Invention
The present invention relates to a method of forming a pattern by photolithography and, more specifically, relates to the alignment during pattern formation.
2. Description of the Related Art
Methods of producing solid-state image pickup devices and display devices, which are optical function devices, according to the related art each include a step of forming a pattern with a blue filter, a black filter, and so on by photolithography. The blue filter, the black filter, and so on are made of materials having a low transmittance for many wavelengths of light. While performing alignment for producing patterns of such blue filter, black filter, and so on, the following problem may occur. Since the light used to detect an alignment mark (hereinafter, referred to as “alignment light”) must transmit through the material of the blue filter, the black filter, and so on, the light reflected from the alignment mark is weak. As a result, it is difficult to detect the alignment mark.
To solve such a problem, in Japanese Patent Laid-Open No. 11-211908, a new alignment mark is formed using a material having a high alignment-light transmittance. A method of performing alignment for forming a pattern on a material having a low light transmittance using such a new alignment mark is described.
Japanese Patent Laid-Open No. 2007-004082 describes a method of detecting an alignment mark after selectively removing a material having low light transmittance above the alignment mark.
Since a new alignment mark is created when the technique described in Japanese Patent Laid-Open No. 11-211908 is used, the new alignment mark includes a shift component with respect to the original alignment. Moreover, since a pattern is formed on the basis of the new alignment mark, the alignment precision is reduced.
According to the technique described in Japanese Patent Laid-Open No. 2007-004082, since the material having a low light transmittance above the alignment mark is removed by photolithography, a step of forming a photo-resist mask pattern for removal is required. Accordingly, with the technique described in Japanese Patent Laid-Open No. 2007-004082, the number of steps in the process increases.
Accordingly, the present invention provides a method of forming a pattern on a material having a low alignment-light transmittance with high alignment precision and in fewer steps. The present invention also provides a structure of an alignment mark that enables the formation of a pattern on a material having a low alignment-light transmittance with high alignment precision and in fewer steps.